The present invention relates to a method of preparing substantially pure polyhalophenoxyphosphazene homopolymers and copolymers. More particularly, the invention relates to a method of preparing such substantially pure polymers which involves preparing the salt or salts of a halogen-substituted phenol or a mixture of a halogen-substituted phenol and an alkyl or alkoxy-substituted phenol, alkanol or fluoroalkanol by reacting said halogen-substituted phenol or mixture with sodium hydride and then reacting the resultant salt or salts with a polydichlorophosphazene polymer.
Methods of preparing polyphosphazene homopolymers or copolymers which contain halophenoxy groups are known in the prior art as illustrated by, for example, U.S. Pat. Nos. 3,370,020; 3,856,712; and 3,883,451. However, the halophenoxy group-containing polyphosphazene homopolymers and copolymers produced by the process described in these patents are not pure polymers, but are in fact polymers containing mixtures of units of differing chemical structures. The reason for this is related to the method of preparing the alkali metal salt of the halogen-substituted phenol employed in such prior processes.
Thus, as described in the aforementioned patents, the polymers are prepared by reacting a polydichlorophosphazene polymer with the alkali metal salt of a halogen-substituted phenol in the case where homopolymers are desired or the alkali metal salts of a mixture of a halogen-substituted phenol and a nonhalogen-substituted phenol in cases where a copolymer is desired. Moreover, as further described in the above patents, the alkali metal salt of the halogen-substituted phenol is prepared by reacting the halogen-substituted phenol with an alkali metal especially sodium. However, the use of an alkali metal to form the alkali metal salt of a halogen-substituted phenol does not result in the production of a single salt but in actuality results in the preparation of a mixture of salts. For example, the use of sodium metal to produce the sodium salt of p-chlorophenol results in the production of a mixture of sodium salts consisting of sodium p-chlorophenoxide and sodium phenoxide. The reason for this is that sodium metal will reduce a proportion of the p-chlorophenol to phenol by means of an electron transfer process.
Accordingly, attempts to prepare a polyhalophenoxyphosphazene homopolymer using an alkali metal salt prepared by such prior processes will, in fact, result in the preparation of a polymer containing a mixture of units of different chemical structures. Hence, for example, attempts to produce a polyphosphazene homopolymer containing p-chlorophenoxy groups in which the sodium salt of p-chlorophenol is produced by reaction with sodium metal as in prior processes will result in the production of a polymer containing a mixture of units of the structure: ##STR1## In essence, the above polymer would be regarded as a copolymer rather than a homopolymer. Similarly, polyphosphazene copolymers containing polyhalophenoxy groups prepared by such processes would also contain mixtures of units in which additional phenoxy groups formed by reduction of halophenol are present.
In contrast to the processes of the prior art, the process of the present invention permits the production of substantially pure polyhalophenoxyphosphazene homopolymers and copolymers which do not contain such additional phenoxy groups.